Figuring out the RPM's of a bullet is actually pretty easy, it's just a matter of converting units.
Say your bullet is doing 3500 feet per second out of a 1 in 8 inch twist barrel and you want to convert to revolutions per minute.
Multiply 3500 feet per second times 60 because there are 60 seconds in one minute. That equals 210,000 feet per minute the bullet is travelling.
Now you've got to figure out how many revolutions the bullet makes in a foot. Since it makes one revolution in 8 inches divide 12 by 8 since there are 12 inches in 1 foot. That equals 1.5 revolutions per foot.
Now since it travels 210,000 ft per minute and it revolves 1.5 times in every foot of travel multiply 210,000 times 1.5. That equals 315,000 RPM that bullet is turning.
So the formula to figure out the RPM of a bullet is: Muzzle velocity X 60 X 12/twist rate = bullet rpm
The higher the rpm the higher the centrifugal force that makes the bullet want to fly apart. Bullet construction is critical to surviving high velocities out of fast twist barrels. I've shot 40 gr. ballistic tips out of my 1-8" twist 22-250 at 4200 fps and they held together just fine at the 378,000 RPM they're turning. If you shoot a lightly constructed hollow point like the speer TNT at those rpm's and you'll probably just see a puff in front of the barrel. It doesn't mean the speer is a bad bullet, it's just not designed for that.
The plastic tipped bullets seem to be able to withstand these extreme rpm's a lot better than hollow point or exposed lead tip bullets.
